The present invention relates to communications receivers, particularly but not exclusively, to zero IF receivers which may be fabricated as integrated circuits. In the present specification "zero IF" or "zero intermediate frequency" is to be interpreted as covering low IF frequencies as well as zero IF.
An advantage of zero IF receivers is that they can be implemented as integrated circuits having very few off-chip components. However, one of the most serious limitations preventing the wider application of such receivers is direct detection caused by inadequate second-order inter-modulation performance. This inadequate second-order inter-modulation performance allows strong input signals occurring anywhere within the bandwidth of the front end of the receiver to effectively bypass all the discrimination which would otherwise be provided by the adjacent channel selectivity filters at the intermediate frequency.